Catalytic hydrocracking is a refining process that uses hydrogen and catalysts at relatively low temperature and high pressures for converting middle boiling points to naphtha, reformer charge stock, diesel fuel, jet fuel, or high-grade fuel oil.

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2.  Hydrocracking catalyst is susceptible to poisoning by
metallic salts, oxygen, organic nitrogen compounds, and
sulfur in the feedstocks.
 The feedstock is hydrotreated to saturate the olefins and
remove sulfur, nitrogen, and oxygen compounds.
 Molecules containing metals are cracked and the metals
are retained on the catalyst.
 Nitrogen and sulfur compounds are removed by conversion
to ammonia and hydrogen sulfide.
 Organic nitrogen compounds are thought to act as
permanent poisons to the catalyst, the ammonia produced
by reaction of the organic nitrogen compounds with
hydrogen does not affect the catalyst permanently
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3.  Presence of hydrogen sulfide in low concentrations acts as a
catalyst to inhibit the saturation of aromatic rings.
 This is a beneficial effect when maximizing gasoline
production as it conserves hydrogen and produces a higher
octane product.
 Hydrogenation reactions, such as olefin saturation and
aromatic ring saturation, take place, but cracking is almost
insignificant at the operating conditions used.
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4.  Necessary to reduce the water content of the feed streams
to less than 25 ppm because, at the temperatures required
for hydrocracking, steam causes the crystalline structure of
the catalyst to collapse and the dispersed rare-earth atoms
to agglomerate.
 Water removal is accomplished by passing the feed stream
through a silica gel or molecular sieve dryer
 On the average, the hydrogen treating process requires
approximately 150 to 300 ft3 of hydrogen per barrel of feed
(27 to 54 m3 hydrogen per m3 feed).
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5.  Composition is tailored to the process, feed material, and
the products desired.
 Consist of a crystalline mixture of silica-alumina with a
small uniformly distributed amount of rare earth metals
contained within the crystalline lattice.
 The silica-alumina portion of the catalyst provides cracking
activity while the rare-earth metals promote
hydrogenation.
 Catalyst activity decreases with use, and reactor
temperatures are raised during a run to increase reaction
rate and maintain conversion.
 The catalyst selectivity also changes with age and more gas
is made and less naphtha produced as the catalyst
temperature is raised to maintain conversion.
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6.  It will take from two to four years for catalyst activity to
decrease from the accumulation of coke and other deposits
to a level which will require regeneration.
 Regeneration is accomplished by burning off the catalyst
deposits, and catalyst activity is restored to close to its
original level.
 The catalyst can undergo several regenerations before it is
necessary to replace it.
 Almost all hydrocracking catalysts use silica-alumina as the
cracking base but the rare-earth metals vary according to
the manufacturer.
 Those in most common use are platinum, palladium,
tungsten, and nickel.
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7.  The severity of the hydrocracking reaction is measured by
the degree of conversion of the feed to lighter products.
 Conversion is defined as the volume percent of the feed
which disappears to form products boiling below the
desired product end point
 The primary reaction variables are reactor temperature and
pressure, space velocity, hydrogen consumption, nitrogen
content of feed, and hydrogen sulfide content of the gases
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8.  Reactor Temperature
 Primary means of conversion control.
 At normal reactor conditions a 20°F (10°C) increase in
temperature almost doubles the reaction rate, but does not
affect the conversion level as much because a portion of the
reaction involves material that has already been converted
to materials boiling below the desired product end point.
 As the run progresses it is necessary to raise the average
temperature about 0.1 to 0.2°F per day to compensate for
the loss in catalyst activity.
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9.  Reactor Pressure
 Effects on the partial pressures of hydrogen and ammonia.
 An increase in total pressure increases the partial pressures
of both hydrogen and ammonia.
 Conversion increases with increasing hydrogen partial
pressure and decreases with increasing ammonia partial
pressure.
 The hydrogen effect is greater, however, and the net effect
of raising total pressure is to increase conversion.
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10.  Space Velocity
 The volumetric space velocity is the ratio of liquid flow
rate, in barrels per hour, to catalyst volume, in barrels.
 The catalyst volume is constant, therefore the space
velocity varies directly with feed rate.
 As the feed rate increases, the time of catalyst contact for
each barrel of feed is decreased and conversion is lowered.
 In order to maintain conversion at the proper level when
the feed rate is increased, it is necessary to increase the
temperature.
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11.  Nitrogen Content
 Hydrocracking catalyst is deactivated by contact with
organic nitrogen compounds.
 An increase in organic nitrogen content of the feed causes a
decrease in conversion.
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12.  Hydrogen Sulfide
 At low concentrations the presence of hydrogen sulfide acts
as a catalyst to inhibit the saturation of aromatic rings.
 This conserves hydrogen and produces a product with a
higher octane number
 Hydrocracking in the presence of a small amount of
hydrogen sulfide normally produces a very low-smoke-
point jet fuel.
 At high hydrogen sulfide levels corrosion of the equipment
becomes important and the cracking activity of the catalyst
is also affected.
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13.  Heavy Polynuclear Aromatics (HPNA)
 HPNA are formed in small amounts from hydrocracking
reactions and, when the fractionator bottoms is recycled,
can build up to concentrations that cause fouling of heat
exchanger surfaces and equipment.
 Steps such as reducing feed end point or removal of a drag
stream may be necessary to control this problem
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14. Leyla Ibrahimli
Chemistry teaching
leylaibrahimli@gmail.com
110411018
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